1. Field of the Invention
The present invention relates to a solid-state imaging element, a method for driving it, and a camera system, and particularly to an amplification type solid-state imaging element such as a CMOS image sensor having an amplification function for each of unit pixels arranged in a matrix form, a method for driving it, and a camera system using amplification type solid-state imaging elements as imaging devices.
2. Description of Related Art
Amplification type solid-state imaging elements, for example, CMOS image sensors have various pixel structures. As an example, there is known a pixel structure having floating diffusion (FD) inside pixels. This pixel structure is advantageous in that sensitivity can be increased because signals are amplified by the floating diffusion. FIG. 18 shows a prior art pixel structure of this type.
In FIG. 18, each of unit pixels 100 arranged in a matrix form includes photogate 101, transfer switch 102, floating diffusion 103, reset transistor 104, amplifying transistor 105, and vertical selection transistor 106. In response to a vertical selection pulse afforded via the vertical selection line 111, the vertical selection transistor 106 selects unit pixels 100 in units of rows, whereby a signal amplified by the amplifying transistor 105 is output to the vertical signal line 112.
By the way, to reduce pixel size requires that the number of elements to constitute a unit pixel 100 is reduced. However, since the pixel structure of a prior art CMOS image sensor described above dictates that three transistors, reset transistor 104, amplifying transistor 105, and vertical selection transistor 106, are used to select the potential of floating diffusion 103 in units of rows for output to vertical signal line 112, a large number of elements are used, hindering reduction of pixel size.